1. Field of the Invention
The present invention pertains to a dielectric structure which serves as a conduit for the flow of heat transfer fluid to an upper surface of an electrostatic chuck. The dielectric structure comprises a dielectric insert which is typically used in combination with at least a portion of a dielectric layer which forms the upper surface of the electrostatic chuck. The dielectric structure prevents breakdown of the heat transfer fluid fed through the electrostatic chuck to its surface to cool a bottom surface of a work piece such as a silicon wafer which is supported upon the upper surface of the electrostatic chuck. The dielectric structure also prevents a semiconductor processing plasma from penetrating into the heat transfer fluid openings in the electrostatic chuck.
2. Brief Description of the Background Art
U.S. Pat. No. 5,350,479 to Collins et al. issued Sep. 27, 1994, and hereby incorporated by reference, describes an electrostatic chuck for holding an article (typically a semiconductor substrate) to be processed in a plasma reaction chamber. The electrostatic chuck includes a metal pedestal coated with a layer of dielectric material which contains a system for distributing a cooling gas upon the upper surface of the electrostatic chuck so that it contacts the bottom of an article supported on that surface. The gas distribution system includes a plurality of intersecting grooves formed entirely in the upper surface of the electrostatic chuck, with small gas distribution holes through intersections of the grooves.
The lifetime of an electrostatic chuck is affected by the presence of the gas distribution holes used to facilitate the distribution of heat transfer gas. In particular, when the electrostatic chuck is subjected to high power RF fields and high density plasmas immediately above the semiconductor substrate, it is possible to have breakdown of the cooling gas due to arcing or glow discharge. Further, since there is a direct, straight line path between the semiconductor substrate supported on the upper, dielectric surface of the electrostatic chuck and an underlying conductive layer of aluminum which forms the pedestal of the electrostatic chuck, arcing can occur along this path. Arcing or glow discharge at the surface of the semiconductor substrate can result in loss of the substrate. In addition, arcing or glow discharge within the gas distribution holes deteriorates the dielectric layer and underlying aluminum layer.
Collins et al. recommends that the aluminum layer beneath the dielectric layer be cut back (away) beneath the dielectric layer immediately adjacent the gas distribution hole to reduce the possibility of arcing across the straight line path from the semiconductor substrate to the aluminum layer. Although this reduces the possibility of arcing, it does not provide the desired isolation of the conductive electrostatic chuck from the process plasma.
U.S. Pat. No. 5,315,473 to Collins et al., issued May 24, 1994, and hereby incorporated by reference, describes methods of improving the clamping force of the electrostatic chuck among other features. In particular, the composition of the dielectric material and the thickness of the dielectric layer are among the critical factors in determining the clamping force. Since it is not yet practical to produce a dielectric layer which is totally flat, there are spacial gaps to be overcome. Generally, the thinner the dielectric layer, the greater the clamping force, all other factors held constant. However, there are practical limitations which limit the ultimate thickness of the dielectric layer. For dielectric layers approximately 1 mil or less in thickness, it has been found that the dielectric material breaks down and loses its insulating properties at voltages required to overcome the spacial gaps between the semiconductor substrate and the upper surface of the electrostatic chuck.
European Patent Application No. 93309608.3 of Collins et al., published Jun. 14, 1994, and hereby incorporated by reference, describes the construction of an electrostatic chuck of the kind disclosed in U.S. Pat. No. 5,350,479 referenced above. The construction of the electrostatic chuck includes grit blasting of the aluminum pedestal, followed by spraying (e.g. plasma-spraying) a dielectric material such as alumina or alumina/titania upon the grit-blasted surface of the aluminum pedestal. Typically the sprayed dielectric thickness is greater than the desired final thickness, by about 15-20 mils (380-508 microns). After the dielectric material has been applied, the thickness is reduced by grinding until it has the desired final thickness, for example, of about 7 mils (180 microns). The upper surface of the dielectric layer is then processed to provide a pattern of heat transfer gas distribution grooves over the surface of the layer. Perforations are created through the dielectric layer to connect the heat transfer gas distribution grooves with gas distribution cavities contained in the pedestal of the electrostatic chuck. In some instances, the perforations in the upper surface of the underlying aluminum pedestal which lead to gas distribution cavities within the pedestal are prepared in advance of application of the dielectric layer. In other instances, the perforations in the upper surface of the aluminum pedestal are prepared simultaneously with the perforations through the dielectric layer.
The cooling gas distribution grooves in the surface of the dielectric layer can be produced using laser micro machining or by using a grinding wheel. The perforations through the dielectric layer are formed using a mechanical drill or a laser. A preferred laser is an excimer UV laser (i.e. a short wave-length, high energy laser) run at a relatively low time averaged power level. This helps reduce the redepositing of drilled aluminum from the underlying thin layer onto the walls of the perforations and onto the surface of the dielectric. Presence of such aluminum can cause arcing across the dielectric layer. The perforations are frequently placed around the outer perimeter of the surface of the electrostatic chuck. For an electrostatic chuck used with an 8 inch (200 mm) silicon wafer electrostatic chuck, the number of such perforations generally ranges from about 150 to about 300. The number of perforations depends on the amount of heat transfer load, and the heat transfer fluid flow rate required to handle this load. Typically the perforations are configured in a ring-like structure around the outer perimeter of the electrostatic chuck. A typical perforation has a diameter which is approximately 0.007.+-.0.001 inch (0.175.+-.0.025 mm).
While micro-drilling through the dielectric layer overlaying the aluminum pedestal to provide the perforations described above provides a satisfactory gas passage, it fails to address the problem of the RF plasma environment that seeks the interface between the dielectric alumina coating and the aluminum substrate. Frequently the underlying aluminum works its way up the sidewalls of the opening(s) in the dielectric layer, leading to arcing and plasma glow within the opening(s). Moreover, depending on the method used to form the perforations, the lower portion of the hole may become a metallic conductor (aluminum) despite the use of a high aspect ratio (depth/diameter) for the gas passage. The removal of machined micro chips slurry from the distribution hole is a difficult task, and is compounded by any migration of aluminum particles up through the dielectric gas distribution hole during drilling and subsequent manufacturing operations such as cleaning of passageways. Presence of machined micro chips slurry is a source of contaminant in the micro electronic environment and may even block the holes in a manner that reduces or stops heat transfer gas flow.
In light of the above, there is a need for a structure which significantly reduces the possibility of breakdown of the cooling gas due to arcing or glow discharge. Further, there is a need for a structure which significantly reduces the possibility of arcing between a semiconductor substrate and the metallic pedestal portion of the electrostatic chuck on which the semiconductor substrate is supported.